The present invention is related to high voltage power supplies for a cathode ray tube. Specific advantage is found in using the present invention in a high voltage power supply for a medical instrument wherein physiological data is displayed on a cathode ray tube.
It is desirable that the high voltage power supply used for a cathode ray tube in a display system fulfill many varied requirements. One of these characteristics is that the high voltage supply exhibit good regulation against load and line changes including quick action to prevent shifting of the image on the display on turn-on and off of functions within the instrument and additionally to prevent undue variation in the overall display size during normal operation. An additional desirable characteristic is that the high voltage supply operates at a frequency beyond twenty kilohertz so that it avoids audible noise. A further desirable feature in a supply is that minimum power dissipation in switching transistors occur to minimize the size and leak from heat sink requirements and thus to reduce the spread of high frequency, high voltage noise to sensitive portions of the overall cathode ray tube circuit. It is known that to include large heat sinking means to transistor devices increases capacitive coupling between the devices, resistors and the heat sink and thus induces the propagation of high frequency and high voltage noise throughout other circuit elements also exhibiting capacitive coupling with a heat sink device.
Additional problems are exhibited in cathode ray tube devices as well as other electronic instruments wherein during power turn-on and turn-off cycles, substantial stresses are placed upon circuit components. These occur due to the instantaneous mode of operation of circuit components during the turn-on and the resultant transients in voltage and current. These fluctuations occur within the various circuits of a system until it achieves a stabilized operating condition. While such a transient period may exist only for tenths of a millisecond or less, the existing transients may be of substantial size and put undue stress upon circuit components.
An additional problem occurring in cathode ray tube devices is caused by internal arcing of the cathode ray tube. It is known by the users of cathode ray tubes that periodic short circuits occur among one or more of the high voltage elements within the cathode ray tube. These are theorized to occur because of a build-up of either contaminants or of high electric field strengths on sharp-edged components. To varying degrees, all cathode ray tubes exhibit discharge properties when the voltage rises on various of the structural elements such as high voltage electrodes and the like and instantaneously discharge to surrounding elements. During the interval of this cathode ray tube arcing, there is a temporary short on the power supply, which places substantial stresses on the various components (transistors, integrated circuits and the like) in the supply.
I have determined that a solution to the foregoing problems is in the use of a self-oscillating fly-back converter with appropriate additional regulation of the output voltage thereof. Through this invention, the aforementioned stresses which may be occurring during the continued operation of the high voltage supply of a CRT display may be overcome to provide long and stable operation.
Conventional methods of accommodating for problems of the type indicated above and especially arcing and the short-circuit type are discussed in detail in the following two publications: "The Destructive Circuit Malfunctions and Corrective Techniques in Horizontal Deflection" by C. F. Wheatley, IEEE Transactions on Broadcast and TV Receivers, July 1965, pages 102-111 and "Flashover in Picture Tubes and Methods of Protecting" by A. Ciuciura, The Radio and Electronic Engineer, March 1969, pages 149-168.
As will be noted from the following detailed description of a preferred embodiment, the present invention involves a technique different and alternative to those described in the above-mentioned publications. It should be noted that the present invention allows a straight-forward, easily implemented technique enabling an inexpensive and uncomplicated instrumentation into a cathode ray tube display, high voltage power supply.